Assessing the efficiency of iron fertilization
on
atmospheric CO2 using an intermediate
complexity ecosystem model of the global
oceanOlivier Aumont1 and Laurent Bopp2
1IPSL / LODyC, Paris, France2IPSL / LSCE, Gif s/ Yvette, France
The Ocean in a High CO2 World
Introduction : The HNLC regions
The Iron hypothesis
Fe
The Iron fertilization experiments
A: IronexI
B: IronexII
C: SOIREE
D: EisenEx
E: SEEDS
F: SOFEX
G: Planktos
H: SERIES
AB
C D
E
F
G
H
Main results
Chlorophyll From a 3 to a 40-fold increase, generally as diatoms
pCO2 A 30 to 90 atm drawdown in surface pCO2
Export Production Contrasting results, generally an increase
DMS An increase
Mitigation of atmospheric CO2
Large scale iron fertilization
Iron fertilization can be used as a means of offsetting the anthropogenic carbone dioxide emission
(Martin et al., 1991)
Previous estimates (modeling studies)
Peng and Broecker, 1991
Joos et al., 1991
Box models Southern Ocean Preindustrial: -17 to -59 atm
Anthropogenic : -64 to -107 atm
Sarmiento and Orr, 1991 OGCM
Nutrient restoring
Global Ocean Preindustrial: -3 to -72 atm
Six and Maier-Reimer, 1993 OGCM
HAMOCC
Southern Ocean Preindustrial: -34 atm
Anthropogenic : -50 atm
Archer et al., 2000 OGCM
HAMOCC
Global Ocean Preindustrial: -50 atm
Ganadesikan et al., 2003 OGCM
Nutrient restoring
Patchy, equatorial Pacific
Low efficiency, < 10% of increase in export production as atmospheric CO2
Questions
Iron fertilization
Can the model simulate the main features of the iron fertilization experiments ?
What is the spatial and temporal variability of the response to fertilization ?
What is the long-term efficiency of the fertilization ?
Outline
1. Model description
2. Patchy iron fertilization
3. long-term iron fertilization on the global scale
Tools : Tools : Models
OPA PISCES
PO43-
Diatoms
MicroZoo
P.O.M
D.O.M
Si
IronNano-phyto
Meso Zoo
NO3-
NH4+
Small Big
Euphotic Layer (10-200m)
Chlorophyll surface concentrationsChlorophyll surface concentrations
Seawifs
(98-03)
PISCES
June January
Iron distribution
Annual mean, surface
Annual mean, 1000m
0.1
0.5
1
1.5
3
5
0.1
0.5
1
1.5
3
5
What limits diatoms growth ?What limits diatoms growth ?
NO3 + NH4 PO4 Fe Si
Iron Fertilization
‘‘ Patchy ’’ Iron Fertilization in the three main HNLC regions
Experimental design
- Iron concentration set to 2 nM in the mixed layer at day 2 and 5
- The model is integrated for 31 days
- Fertilization applied over only one grid box
Iron Fertilization : The Southern Ocean (1)
2. Small response (Chl < 0.7 mg Chl m-3)
3. Moderate response (0.7 < Chl< 2.5 mg Chl m-3)
4. Strong response (2.5 mg Chl m-3 < Chl )
1. Blooming conditions (Chl > 1.5 mg Chl m-3)
0
2.0
4.0
6.0
0
20
40
60
80
Chla
Diatoms relative abundance
The Southern Ocean
pCO2 (atm)
Export ()
-100
-60
-20
0
0
40
80
120
2. Si limitation, Si initial < 6 umol L
3. Mixed layer depth > 30 m, macronutrient replete
4. Favorable conditions, strongly iron limited
1. Stratification, ice retreat
Why such responses ?
The Southern Ocean : Comparison with data
pCO2 (atm)
Diatoms relative abundance
Chla (mg Chl m-3)
SOFEX South
SOFEX North
SOIREE
Seasonal evolution
January February
July November
Iron Fertilization : The equatorial Pacific
Diatoms relative abundance
Chla (mg Chl m-3)
IRONEX II
pCO2 (atm)
export (%)
Iron Fertilization : everywhere & 50 yr longChanges in Diatoms Relative Abundance
+1
-1
Export Production (GtC/yr)
7
8
-10 0 10 20 30 40 50
Years
Fe Fertilization
Increase of Export Production
+50
+5
-5
-50
(gC/m2/yr)
+4
-4
+0.2
+1
-0.2
-1
Changes in Chla (mg Chl m-3)
-10 0 10 20 30 40 50
Years
Fe Fertilization
Atmospheric pCO2(atm)
Carbon Flux(PgC/yr)
1
0
0
Impact on atmospheric pCO2
Preindustrial conditions
-10
-4 atm in 10 yr
Fe Fertilization
0.5
0
0
-10
7
8
Atmospheric pCO2(atm)
Carbon Flux(PgC/yr)
Export(PgC/yr)
-1.8 atm in 50 yr
-10 0 10 20 30 40 50
-8 atm in 50 yr
>80% due to Southern Ocean
Why such a small efficiency ?
Nutrient Limitation of Diatoms Growth
NO3 / NH4 PO4 Fe Si
Control
Fe Fert.
Light limitation
-10
-2
2
-1
1
NO3 (mol/L)NO3 (mol/L)
1
4
10
20
Iron Fertilization :
Implications for the Sulfur Cycle
Changes in Surface DMS Concentrations
Export Production
Atmospheric pCO2
Carbon Flux
-8 ppm in 50 yr
-5
-0.5
+0.5
+5
nM
7
8
1
0
0
-10
DMS Flux(TgS/yr)-15 %
-10 0 10 20 30 40 50
22
26
Conclusions
Patchy Iron fertilization :
The model roughly captures the main features of in situ iron fertilization experiments, except in the North Pacific.
In the Southern Ocean, the response depends highly on the location and the time period of the iron release. Main controlling factors are Si concentrations, the mixed layer depth, and the status of the ecosystem.
The favorable season extends from November to March.
Large-scale Iron fertilization :
Very low efficiency : only 8 ppmv drawdown in atmospheric pCO2 after 50 years.
Iron fertilization should be done continuously to keep the additionally stored CO2 within the ocean.
Possible drawbacks : N2O production, extension of the anoxic regions, changes in the fisheries, possible decrease in DMS production, …
Diatoms relative abundance : vs DataDiatoms relative abundance : vs Data
1
0.8
0.6
0.4
0.2
0
Data from Gregg et al. 2003
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
60°S
40°S
20°S
0°
20°N
40°N
60°N
80°N
Iron Fertilization : The North Pacific
Diatoms relative abundance
Chla (mg Chl m-3)